Scientific Method —

Turbulence ahead: Interstellar wind changes direction, blows faster

Shift in interstellar wind reveals our changing galactic environment.

Artist's conception of the Sun's motion through the Local Interstellar Cloud (LIC). The motion of gas in the cloud means that the motion of particles through the Solar System has changed over the last 40 years, as observed by a number of different spacecraft.

Interstellar space—the region between stars in our galaxy—is fairly empty. There are still enough atoms in that space to produce a measurable effect as the Sun orbits the galactic center, however. The flow of these atoms, known as the interstellar wind, provides a way to study interstellar gas, which moves independently of the Sun's motion.

A new analysis of 40 years of data showed that the interstellar wind has changed direction and speed over time, demonstrating that the environment surrounding the Solar System changes measurably as well. Priscilla Frisch and colleagues compared the results from several spacecraft, both in Earth orbit and interplanetary probes. The different positions and times in which these instruments operated revealed that the interstellar wind has increased slightly in speed. Additional measurements revealed that the flow of atoms has shifted somewhere between 4.4 degrees and 9.2 degrees. Both these results indicate that the Sun is traveling through a changing environment, perhaps one shaped by turbulence in interstellar space.

The properties of the Solar System are dominated by the Sun's gravity, magnetic field, and the flow of charged particles outward from its surface. However, a small number of electrically neutral particles—mostly light atoms—pass through the Solar System. These particles are part of the local interstellar cloud (LIC), a relatively hot region of space governed by its internal processes.

Neutral helium is the most useful product of the interstellar wind flowing through the Solar System. Helium is abundant, comprising roughly 25 percent of all interstellar atoms. In its electrically neutral form, helium is largely unaffected by magnetic fields, both from the Sun and within the LIC. The present study also considered neutral oxygen and nitrogen atoms, which are far less abundant, but more massive and therefore less strongly jostled even than helium.

When helium atoms flow through the Solar System, their paths are curved by the Sun's gravity depending on how quickly they are moving. Slower atoms are more strongly affected than faster ones, so the effect is a cone of particle trajectories. The axis of that focusing cone is the dominant direction of the interstellar wind, while the width of the cone indicates how much variation in particle speeds is present, a measure of the speed and turbulence in the LIC.

The interstellar wind was first measured in the 1970s by missions such as the Mariner 10 (which flew by Venus and Mercury) from the United States and the Prognoz 6 satellite from the Soviet Union. More recently, the Ulysses spacecraft in solar orbit, the MESSENGER probe studying Mercury, and the IBEX (Interstellar Boundary EXplorer) mission collected data from several perspectives within the Solar System.

When the authors of the present study collated the data from 1972 through 2011, they found a noticeable change in the direction of the interstellar wind in each successive decade of observation. Compared with the direction of the Sun's motion through the galaxy, the flow of particles has shifted to be more directly against us. This directional change is by an estimated 6.8 degrees, albeit with a relatively large error of ±2.4 degrees. However, even with that statistical uncertainty, the possibility of no change in direction is ruled out to a significant degree.

Comparing the Ulysses and IBEX data, the researchers also found an increase in wind speed between 2.6 and 3.6 kilometers per second (5,800 and 8,100 miles per hour). Both this and the change in direction indicate a change in the environment of the LIC as the Sun moves through it. Since our immediate interstellar environment is a cloud of gas (though a diffuse one), measurements like this are useful for determining its detailed behavior—and helping astronomers understand general properties of gas clouds in the galaxy.

Finally a fresh argument against man made global warming. My well was almost dry.

It's the winds! The interstellar winds!

I know you're being sarcastic, but I was actually going to post a question asking is this might be responsible for some of the warming. I debated posting it because it would lead to the same old flame wars on here, but since you opened the door, I'm actually curious if this could be another part of the equation.

Finally a fresh argument against man made global warming. My well was almost dry.

It's the winds! The interstellar winds!

I know you're being sarcastic, but I was actually going to post a question asking is this might be responsible for some of the warming. I debated posting it because it would lead to the same old flame wars on here, but since you opened the door, I'm actually curious if this could be another part of the equation.

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Finally a fresh argument against man made global warming. My well was almost dry.

It's the winds! The interstellar winds!

I know you're being sarcastic, but I was actually going to post a question asking is this might be responsible for some of the warming. I debated posting it because it would lead to the same old flame wars on here, but since you opened the door, I'm actually curious if this could be another part of the equation.

No. This "wind" is still so diffuse it would register as a vacuum on earth.

Thank you. It restores my faith in these forums that someone took the time to answer the question instead of just downvoting my post.

Well, I edited it to remove further ambiguity. Hopefully you're not offended as I did not intend to mock you. I just want to emphasize how miniscule this phenomenon is. It's REALLY tiny. You could stick a sheet of paper in solar orbit and this wind might push it an inch a year.

Well, I edited it to remove further ambiguity. Hopefully you're not offended as I did not intend to mock you. I just want to emphasize how miniscule this phenomenon is. It's REALLY tiny. You could stick a sheet of paper in solar orbit and this wind might push it an inch a year.

Not offended at all. I'm no-where near an expert on matters of global warming or interstellar physics, but I find the science for both extremely interesting. I just usually avoid asking questions about anything remotely related (or possible relation) to global warming on this site because the signal to noise ratio is out of whack on both sides. Thanks again.

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Well, technically it must have a measurable effect here because we measured it .

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Well, technically it must have a measurable effect here because we measured it .

Slower atoms are more strongly affected than faster ones, so the effect is a cone of particle trajectories. The axis of that focusing cone is the dominant direction of the interstellar wind, while the width of the cone indicates how much variation in particle speeds is present, a measure of the speed and turbulence in the LIC.

Wouldn't the dominant direction be the fastest (least perturbed by the sun's gravity) particle's direction, which would be along one edge of the cone, not along its center axis?

This would be quite interesting if anyone put stock in the Nemesis hypothesis. If Nemesis existed, such a change could be the result of a "bow wave".

It would be awesome if we could fire off a dozen or more cheap probes over the course of a year( or whatever time it takes to get the trajectories worked out with appropriate slingshots) to get them all exiting the solar system at different points. Two probes mapping the solar system boundary just seems anemic.

I've been starting to wonder if it really is quite as empty as I used to think. I recently found out the Oort cloud may extend out to 2 or 3 light-years from the sun. If so, it would overlap or at least approach colliding with the Oort clouds of neighboring stars, since the nearest star is "only" 4.2 ly away.

I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

"I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

Not dense, but clumpier. The total estimated mass of everything in the Oort cloud is still only at most a few tens of Earth masses. This is spread over a shell surrounding the Sun spherically and some 15 trillion kilometers (10 trillion miles) thick. Talk about sparse. Assuming everything out there is essentially water ice (a good approximation) that makes for 5.98x10^51 water molecules assuming 30 Earth masses of material (probably way more than is there). The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

THe average density of the interstellar medium is one atom per cubic centimeter. A water molecule is 3 atoms, so it is almost 800 times denser in terms of number of atoms.

Now I need to copy all this so I can use it when I teach my astronomy students about the Oort cloud!

I was running the Universe sandbox the other day and didn't realise that even within our galactic neighbourhood stars are moving every which way - at least relative to each other, even if they are rotating around the galactic core. If the sandbox is right it also suggests some stars will be significantly closer to ours within the next ten thousand years. I wonder what it would look like in the sky?

On a more local note it puts Australia's federal election to decide our government for -three- years in perspective.

I was running the Universe sandbox the other day and didn't realise that even within our galactic neighbourhood stars are moving every which way - at least relative to each other, even if they are rotating around the galactic core. If the sandbox is right it also suggests some stars will be significantly closer to ours within the next ten thousand years. I wonder what it would look like in the sky?

On a more local note it puts Australia's federal election to decide our government for -three- years in perspective.

Grain of rice vs Betelgeuse?

Alpha Centauri will reach a minimum distance of about a parsec (3.26 light-years).

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Well, technically it must have a measurable effect here because we measured it .

</obnoxious mode>

...It doesn't effect the sun or the earth...

First, I expect the word you wanted was affect, or maybe aspect? Or perhaps the word was shrubbery, one that looks nice.

Second, these particles have momentum, if they impact the earth they will transfer that momentum, no? It may be infinitesimal, but it should have some extremely small effect I would think.

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Well, technically it must have a measurable effect here because we measured it .

</obnoxious mode>

...It doesn't effect the sun or the earth...

First, I expect the word you wanted was affect, or maybe aspect? Or perhaps the word was shrubbery, one that looks nice.

Second, these particles have momentum, if they impact the earth they will transfer that momentum, no? It may be infinitesimal, but it should have some extremely small effect I would think.

Third, there is no number three.

It has a calculable effect, not necessarily physical effect due to quantum fluctuations. Kind of like calculating the acceleration I produce on the Earth (on the order of 10^-21 m*s^-2). Calculable, but swamped by quantum effects.

No. And I can't emphasize "no" enough. It's more like "no,no,no,no,no, no, nope-ity-nope, no" This "wind" is still so diffuse it would register as a vacuum on earth. It doesn't effect the sun or the earth to any measurable degree, and even if it did, anthropogenic carbon is a far, far, far, far bigger factor.

Well, technically it must have a measurable effect here because we measured it .

How do these probes physically measure the direction of a neutral particle? I've seen the gels which are returned by various probes, where you can look at this translucent slab and see the actual particle tracks within them. But for probes that don't return to Earth, how is it done?

And some counter-advocacy; most of the experimental evidence for cloud formation being affected (or effected, both are valid here!) by cosmic ray incidence are to do with high velocity charged particles, and this article is about the incidence rate of neutral particles at lower velocities.

"I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

Not dense, but clumpier. The total estimated mass of everything in the Oort cloud is still only at most a few tens of Earth masses. This is spread over a shell surrounding the Sun spherically and some 15 trillion kilometers (10 trillion miles) thick. Talk about sparse. Assuming everything out there is essentially water ice (a good approximation) that makes for 5.98x10^51 water molecules assuming 30 Earth masses of material (probably way more than is there). The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

THe average density of the interstellar medium is one atom per cubic centimeter. A water molecule is 3 atoms, so it is almost 800 times denser in terms of number of atoms.

Now I need to copy all this so I can use it when I teach my astronomy students about the Oort cloud!

If I follow your maths properly, you are saying the interstellar medium is *more dense* than the Oort Cloud?

>The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

"I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

Not dense, but clumpier. The total estimated mass of everything in the Oort cloud is still only at most a few tens of Earth masses. This is spread over a shell surrounding the Sun spherically and some 15 trillion kilometers (10 trillion miles) thick. Talk about sparse. Assuming everything out there is essentially water ice (a good approximation) that makes for 5.98x10^51 water molecules assuming 30 Earth masses of material (probably way more than is there). The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

THe average density of the interstellar medium is one atom per cubic centimeter. A water molecule is 3 atoms, so it is almost 800 times denser in terms of number of atoms.

Now I need to copy all this so I can use it when I teach my astronomy students about the Oort cloud!

If I follow your maths properly, you are saying the interstellar medium is *more dense* than the Oort Cloud?

Therefore the interstellar medium is almost 800 times denser than the Oort cloud. Surely some mistake - shouldn't this be the other way around?

Nope. The Oort cloud is a region that is gravitationally bound to the Sun. The heliopause (where the solar wind stops and the interstellar medium is uninfluenced by the Sun's emissions) is anywhere from 100-250 AU out. The inner edge of the Oort cloud is estimated to be around 1000 AU out. THe stuff in the Oort cloud can't be directly observed yet. Our telescopes are just not good enough yet to spot an object a few kilometers across that far out moving so slow.

And as far as the density, it is much less dense overall, but is clumped into larger bodies than the interstellar medium. That was my point. It's a fun way to look at it. I hadn't done the calculations out before. The Sun's gravity holds sway over a very large volume. That's the point.

"Neutral helium is the most useful product of the interstellar wind flowing through the Solar System. Helium is abundant, comprising roughly 25 percent of all interstellar atoms. In its electrically neutral form, helium is largely unaffected by magnetic fields, both from the Sun and within the LIC. The present study also considered neutral oxygen and nitrogen atoms, which are far less abundant, but more massive and therefore less strongly jostled even than helium.

Interesting. I would have thought the opposite, considering atomic oxygen or nitrogen would be paramagnetic.

"I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

Not dense, but clumpier. The total estimated mass of everything in the Oort cloud is still only at most a few tens of Earth masses. This is spread over a shell surrounding the Sun spherically and some 15 trillion kilometers (10 trillion miles) thick. Talk about sparse. Assuming everything out there is essentially water ice (a good approximation) that makes for 5.98x10^51 water molecules assuming 30 Earth masses of material (probably way more than is there). The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

THe average density of the interstellar medium is one atom per cubic centimeter. A water molecule is 3 atoms, so it is almost 800 times denser in terms of number of atoms.

Now I need to copy all this so I can use it when I teach my astronomy students about the Oort cloud!

If I follow your maths properly, you are saying the interstellar medium is *more dense* than the Oort Cloud?

Therefore the interstellar medium is almost 800 times denser than the Oort cloud. Surely some mistake - shouldn't this be the other way around?

Nope. The Oort cloud is a region that is gravitationally bound to the Sun. The heliopause (where the solar wind stops and the interstellar medium is uninfluenced by the Sun's emissions) is anywhere from 100-250 AU out. The inner edge of the Oort cloud is estimated to be around 1000 AU out. THe stuff in the Oort cloud can't be directly observed yet. Our telescopes are just not good enough yet to spot an object a few kilometers across that far out moving so slow.

And as far as the density, it is much less dense overall, but is clumped into larger bodies than the interstellar medium. That was my point. It's a fun way to look at it. I hadn't done the calculations out before. The Sun's gravity holds sway over a very large volume. That's the point.

Thanks for your reply. I'm sorry it still isn't clear to me. The ISM is the matter that fills the interstellar space between the stars in a galaxy. These distances could be anywhere from next door at the galactic core to, at a guess, several thousand light years at the outer margins of the galaxy. I'm with you that the ISM starts 100-250 AU out from our local star, and the Oort starts a lot further out, but the ISM keeps going, way out past where the Oort ends, way past that, out to where ISM shades into the intergalactic space - however many thousands of light years out that is. The ISM covers a far, far greater volume than the Oort. So how can the Oort be less dense than the ISM?

"I know the Oort cloud is pretty damn sparse, but it's a lot denser than I used to think the interstellar medium is.

Not dense, but clumpier. The total estimated mass of everything in the Oort cloud is still only at most a few tens of Earth masses. This is spread over a shell surrounding the Sun spherically and some 15 trillion kilometers (10 trillion miles) thick. Talk about sparse. Assuming everything out there is essentially water ice (a good approximation) that makes for 5.98x10^51 water molecules assuming 30 Earth masses of material (probably way more than is there). The volume of the Oort cloud is about 1.4x10^55 cubic centimeters. This gives an average of 0.0004266 molecules per cubic centimeter (or 1 molecule per 2,344 cubic centimeters or 2.3 liters). This is the rough equivalent of one water molecule per 2 liter bottle!

THe average density of the interstellar medium is one atom per cubic centimeter. A water molecule is 3 atoms, so it is almost 800 times denser in terms of number of atoms.

Now I need to copy all this so I can use it when I teach my astronomy students about the Oort cloud!

If I follow your maths properly, you are saying the interstellar medium is *more dense* than the Oort Cloud?

Therefore the interstellar medium is almost 800 times denser than the Oort cloud. Surely some mistake - shouldn't this be the other way around?

Nope. The Oort cloud is a region that is gravitationally bound to the Sun. The heliopause (where the solar wind stops and the interstellar medium is uninfluenced by the Sun's emissions) is anywhere from 100-250 AU out. The inner edge of the Oort cloud is estimated to be around 1000 AU out. THe stuff in the Oort cloud can't be directly observed yet. Our telescopes are just not good enough yet to spot an object a few kilometers across that far out moving so slow.

And as far as the density, it is much less dense overall, but is clumped into larger bodies than the interstellar medium. That was my point. It's a fun way to look at it. I hadn't done the calculations out before. The Sun's gravity holds sway over a very large volume. That's the point.

Thanks for your reply. I'm sorry it still isn't clear to me. The ISM is the matter that fills the interstellar space between the stars in a galaxy. These distances could be anywhere from next door at the galactic core to, at a guess, several thousand light years at the outer margins of the galaxy. I'm with you that the ISM starts 100-250 AU out from our local star, and the Oort starts a lot further out, but the ISM keeps going, way out past where the Oort ends, way past that, out to where ISM shades into the intergalactic space - however many thousands of light years out that is. The ISM covers a far, far greater volume than the Oort. So how can the Oort be less dense than the ISM?

Remember that density is mass divided by volume. The Oort loud just doesn't have that much stuff in it. Let's calculate the total mass of the interstellar medium given its average density of 1 atom per cubic centimeter. In the interstellar medium about 89% of atoms are hydrogen, 9% are helium, and the remaining 2% of atoms are heavier elements than hydrogen or helium. Note these are averages - there are regions far denser and regions less dense. A spread of 10 or 12 orders of magnitude separate the most from least dense regions, but the average will do for our rough calculation. Again, to make things simple, lets model our galaxy as a very squat cylinder. It is roughly 100,000 light years in diameter but only about 1000 light years thick. This gives a volume in cubic centimeters of about 6.65x10^66 cubic centimeters. Taking the abundance of hydrogen to be 0.89 atoms per cc, this gives a mass of hydrogen of 9.83x10^39 kg, or 4.94x10^9 solar masses of just hydrogen! Doing the same calculation for helium we get about a billion (10^9) solar masses of helium, since helium is 4 times the mass of hydrogen that partially makes up for the lower numerical abundance.

So the net result of this is that there is only about 30 Earth masses of material in the Oort cloud lumped into kilometer or so sized chunks. Spread this out over the volume of the solar system and it is a low density. On the other hand, there are some 6 billion solar masses of hydrogen and helium spread out over the volume of the galaxy. The Sun is 300,000 times the mass of the Earth, so that is 60 trillion times the amount of material than in the Oort cloud. From my previous post, the volume of the Oort cloud is 1.4x10^55. That's only 475 billion times the volume.

This is what I try and caution my students with. Astronomy deals with some really mind boggling numbers. Then when you calculate things with them, such as density, you divide huge numbers by nearly equally huge numbers and can come up with small numbers that don't make sense at first blush!

Finally a fresh argument against man made global warming. My well was almost dry.

It's the winds! The interstellar winds!

I know you're being sarcastic, but I was actually going to post a question asking is this might be responsible for some of the warming. I debated posting it because it would lead to the same old flame wars on here, but since you opened the door, I'm actually curious if this could be another part of the equation.

No. This "wind" is still so diffuse it would register as a vacuum on earth.

Thank you. It restores my faith in these forums that someone took the time to answer the question instead of just downvoting my post.

Many of the critters on Ars (but far from most in my opinion) act as if they not understand that the voting system is asymmetric. They seem to vote down comments containing ideas or opinions they disagree with even though the comment is worth being posted even if only because of the informed reply it will obtain.

What I regret the most about Ars is that voting signage does not make this explicit. I can't understand that the Ars team is fine with a system with such a high rate of false positive. There is a clear cognitive dissonance here with Ars reporting about such systems in other fields.

Sorry for the off topic post but unless the Ars UI makes it simpler I won't bother going through the hoops of the official moderating complaint system (don't make me think.